Discharge lamps, and particularly metal-halide discharge lamps and sodium high-pressure discharge lamps frequently use ceramic discharge vessels or arc tubes. Use of ceramics for the arc tube improves the color rendition of the lamp. Typical power ratings of such lamps are in the range of between about 50 to 250 W.
Sodium high-pressure discharge lamps frequently use ceramic discharge vessels or arc tubes made of aluminum oxide, Al.sub.2 O.sub.3, to which minor amounts of additives of other oxides, particularly MgO can be added. In constructions of this type it has been customary to fit a niobium tube to the ends of the, usually elongated, discharge vessel, to form an electrical connection. The niobium tube is fitted into a plug of ceramic material. Niobium has the specific characteristic that its thermal coefficient of expansion is a good approximation of that of the Al.sub.2 O.sub.3 ceramic. Both materials have a thermal coefficient of expansion of about 8.times.10.sup.-6 K.sup.-1.
U.S. Pat. No. 4,376,905, Kerekes, describes a high-pressure discharge lamp of relatively high power, for example in the range of about 400 W. The higher power rating of the lamp requires electrical connection into the interior of the arc tube capable of carrying the higher current. To accommodate this higher current, a plurality of niobium wires are used, having diameters of at the most 0.6 mm. Dangerous stresses between the tube and the plug due to heat are thereby avoided.
U.S. Pat. No. 5,075,587, Pabst et al, assigned to the assignee of the present application, and the disclosure of which is hereby incorporated by reference, describes a high-pressure discharge lamp in which the discharge vessel is made of aluminum nitride, AlN. Solid tungsten pins are used to conduct the electrical current. The thermal coefficient of expansion of the tungsten pins or rods is 5.times.10.sup.-6 K.sup.-1, which is a good approximation of that of the AlN. Use of tungsten has the additional advantage that this material is resistant to the highly corrosive effect of metal halides which can be used as a fill in the lamp. Niobium, however, is not resistant to the corrosive effect of a fill in an arc tube.
It has been tried to increase the corrosion resistance even when using niobium in the region of the seal. In metal-halide lamps, having discharge vessels or arc tubes made of Al.sub.2 O.sub.3, it has been tried to protect the niobium, for example by a recessed fit, or by protective layers, see European Published Patent Application EP-A 0 472 100. This is an expensive way, and difficult to carry out. Use of electrically conductive cermets as end plugs, as described, for example, in European Patent EP-A 0 142 202, does not solve the problem either. The end plugs are more corrosion-resistant than niobium; however, during the operation of the lamp, and due to heating, microfissures occur in the cermet which, conducting electrical energy into the interior of the discharge lamp, acts as an ohmic resistance. These fissures have a tendency to spread and, finally, cause leakage of the originally sealed connection.